change AddUser method
Merge branch 'master' of github.com:umsatz/go-aqbanking
add support for custom aqbanking dirs

DefaultAQBanking() will return a loaded aqbanking instance with
~/.aqbanking as the default directory, and
NewAQBanking(name, location) returns a aqbanking instance configured
differently...
add a note about aqbanking parallalization